Bring a typename into scope from a templated base class

John Fine
Fri Sep 26 02:20:00 GMT 2008

I am porting some code into gcc 4.1.2 from a much more permissive 
compiler.  I need to bring many names into the scopes of a templated 
classes from templated base classes.  (In the more permissive compiler 
they are in scope without extra effort).

For function names, a "using" declaration seems to work (though I've 
only tried a few examples so far).

For typenames, I've seen other code that uses "using typename".  But I 
can't get that to work and can't understand the C++ standard well enough 
to know why it doesn't work.

So here is a minimal example.  Put the following in a .cpp file and 
compile with -c to see the error.  (The compiler can't parse "iterator 
it" because it doesn't know that "iterator" is a typename.

template<class T>
struct A {
typedef T* iterator; };

template<class T>
struct B : A<T> {
using typename A<T>::iterator;
// typedef typename A<T>::iterator iterator;
iterator it; };

B<int> x;

Is there a correct way to do what I want "using typename" to do?
If I use the "typedef typename" instead of the "using typename", it 
makes this example work, but it doesn't quite mean the same thing.  I'll 
use that if there isn't a way to make "using typename" work.  But I'd 
like to first understand the problem.

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